Article-counting apparatus

ABSTRACT

A counter control circuit which requires two separate signals generated by first and second sensors to cause the counter to register a count. The circuit comprises a relay controlled by a thyristor-type semiconductor switch which is turned on and off by signals from the two sensors, and means controlled by the relay for operating the counter.

O United States Patent ['72] inventor Clement F. Cox (56] References Cited Bronx, NY. UNITED STATES PATENTS 830585 2,730,301 1/l956 Beamish 235/98 [22] FM 3126522 3/1964 Fieser 340/39 [45] Patented Man9l97l [73] Assignee Industrionics Controls, Inc. ima y xamine Maynard R. Wilbur Assistant Examiner-Robert F. Gnuse New York, NY.

[54] ARTICLE-COUNTING APPARATUS SHUTTLE SWITCH Attorney-Schiller & Pandiscio ABSTRACT: A counter control circuit which requires two separate signals generated by first and second sensors to cause the counter to register a count. The circuit comprises a relay controlled by a thyristor-type semiconductor switch which is turned on and off by signals from the two sensors, and means controlled by the relay for operating the counter.

PATENTEU m e nan 3; 559,680

' saw 1 0F 2 \II I ATTORNEY.

lu l 66 78 7 M I a 36 31i l yl gl 30 CLEMENT F COX INVENTOR.

ARTICLE-COUNTING APPARATUS This invention relates to apparatus for counting articles and more particularly to a new and improved control system for counters.

One object of this invention is to provide a new and improved system for counting articles processed by cyclically operating machines.

Another object is to provide a new and reliable system for counting workpieces as they are being fed to, discharged from, or positioned with respect to a cyclically operating machine of the type adapted to perform one or more operations such as pressworking, stamping, punching, diecutting, drilling, grinding, etc.

A more specific object is to provide anew and improved two-signal" control system for a counter whereby the counter is prevented from performing a counting operation until the control system has been properly conditioned by two separately generated control signals.

A further specific object is to provide a single, inexpensive and highly accurate system for counting workpieces processed by a cyclically operating machine.

The foregoing objects and other objects disclosed or rendered obvious in this specification are attained by the combination of a counter control circuit which requires two separately generated input signals in order to cause a given counter to perform a counting operation, means including a sensor for producing a first signal directly or at least indirectly in response to movement or positioning of a workpiece, and means for producing a second signal in response to cyclic operation of the machine with which the counter is associated. In describing a signal as being produced at least indirectly in response to a workpiece, it is contemplated that the sensor is operable by some moving part of the machine or associated equipment or mechanism which can move. to the point of operating the sensor only when the workpiece is being fed, discharged or positioned according to normal machine operation. The counter control circuit includes a relay for coupling input pulses to the counter and thyristor-type means in the energizing circuit for the relay solenoid for preventing an erroneous count when the one or the other of the control signals is continuously applied.

Further details and other advantages of the invention are provided by the following description which is to be considered together with the accompanying drawing wherein:

llG. l is a perspective view of a machine embodying the present invention;

FIG. 2 is a perspective view on an enlarged scale of a portion of the machine of FIG. 1; and

H6. 3 is a diagram of a preferred embodiment of a counter control system constructed according to this invention and used with the machine of FIG. 1.

Turning now to FIG. 1, there is shown a press 2 having a frame 4 which carries a ram 6 that is mounted to move down and up with respect to a bed 8 in response to operation of an electrical motor iii which drives the ram through a shaft 12 rotatabiy mounted to frame 4 and conventional intermediate drive mechanism (not shown). Such intermediate mechanism is not shown since it forms no part of the present invention and is well known in the art of presses. In the usual case, shaft 12 is connected to motor by way of a clutch and such intermediate mechanism inciudes an eccentric mounted on shaft 312 and connected to the ram through a connecting rod so that the ram operates through one complete down and up cycle for each revolution of shaft 112. Bed 8 carries a guide 16 adapted to support a workpiece 18. The ram 6 carries a too] 20 designed to operate on a workpiece supported by guide 16. The machine also includes means for stripping a finished workpiece from the guide 16. Such stripping means are shown schematically at 22. since it forms no part of the invention. The work stripper typically comprises a pneumatic or hydraulic actuator 24 which is adapted to raise an ejection pin or pins (not shown) for stripping the workpiece from the guide 16.

Discharge of the stripped workpiece is, achieved by means of a jet of air delivered on command by an air nozzle 26. The discharged parts fall onto a metal chute 30 which delivers them to a collecting receptacle 32.

In accordance with this invention, the press is provided with means for counting finished parts as hereinafter described. In this connection, the chute is provided with a bracket 36 on which is mounted a connector block 38 carrying a resilient electrically conductive probe 40 that extends down to the level of the floor 42 of the chute. The floor 42 is provided with a hole 44 located so that the probe cannot touch the floor. The connector has one or more terminals 46 which are coupled internally of the connector to the probe 40. In the illustrated case, one of the terminals 46 is fitted with an insulated connecting cable 48 for coupling the probe to the counting circuit. The chute is dimensioned so that each finished part discharged from guide 16 must encounter and deflect the probe 40 before falling into the receptacle 32. Each time the probe is encountered by a finished part, it is effectively electrically connected by the part to the chute which is at ground potential. Hence the probe and chute together comprise switch means. As an alternative measure, the connector block 38 could be replaced by a normally open switch with an actuating arm that is positioned like probe 40 so that the switch is closed each time and for so long as its actuating arm is displaced or defected by a workpiece on the chute.

Further in accordance with this invention, the shaft 12 is fitted with a cam 50 having a lobe 52 whose periphery extends over a predetermined angle. Mounted on the. frame is a switch. 54 having a pivotal operating arm 56 carrying a roller 58 that.

rides on the periphery of the cam. Switch 54 is hereinafter termed the shuttle switch since its operation is according to the up and down or shuttle movement of ram 6. The switch 54 is normally open but closes when the roller 58 is engaged by the lobe on cam 50, the lobe acting to cam arm 56 in a direction to close the switch. The switch is coupled to the counting circuit by a suitable insulated connecting cable 60. As an alternative measure, switch 54 and arm 56 could be replaced by a connector block and a probe similar to connector block 38 and probe 40 with the probe disposed to contact only the lobe of cam 50. Hence the probe would provide a ground connection by way of cam 50 only during the period that it is engaged by lobe 52.

In this preferred embodiment, the remainder of the counting circuit is mounted within a case 62 attached to the frame of the press. The case 62 has one or more apertures 64 which serve as windows for the display units of a solenoid-actuated mechanical counter and also an indicator bulb 66 connected as hereafter described. Case 62 has terminals 68 for connecting a power cable (not shown) and also the electrical connecting cables 48 and 66.

Turning now to FIG. 3, the counting circuit mounted within the case 62 comprises a pair of power input terminals 64 across which are connected the primary coils of two input transformers 66 and 68. One end of the secondary coil of transformer 66 is connected in series with a diode 70, a resistor 72, the operating coil 74 of a solenoid relay, and one of the terminals '1', of a triac 76. The other terminal T of the triac is connected to the opposite end of the secondary coil of transformer 66 which is also connected to ground as shown and to one side of a capacitor 78. The other side of capacitor 78 is connected to the junction of diode 70 and resistor 72. A second diode 80 is connected across operating coil 74. The junction of coil 74 and triac terminal T is connected to a stationary relay contact 82 which together with a stationary contact 84 and a moveable contact 86 forms a first double throw switch unit 88. Contact 84 is unconnected. Moveable contact 86 is connected to shuttle switch 54 which, as mentioned earlier, is operable to provide a ground connection as indicated in broken lines in FIG. 2. Terminal T of the triac, also is connected by way of a resistor 90 to the probe 46 which as mentioned earlier, is operable to provide a ground connection as indicated in broken lines in FIG. 2. The gate electrode G of the triac also is connected to probe 40, but by way of another resistor 92.

The secondary coil of transformer 68 is connected across the input terminals of a full wave rectifier 94 made up of four diodes 96, 98, 100 and 102 connected as shown. One end of the transformers secondary is connected between the anode of diode 96 and the cathode of diode 102, while its other end is connected between the anode of diode 98 and cathode of diode 100. The output from rectifier 94 is taken across the junctions of diodes 96v and 98 and the junction of diodes 100 and 102. The latter junction is connected to the moveable contact 106 of a second double throw switch unit 104. The latter has two stationary contacts 108 and 110, the former connected to an input terminal of a solenoid operated mechanical counter 11 1, and the latter connected to one side of indicator bulb 66. The circuits to the counter 111 and bulb 66 are completed by a common lead 112 which is connected to the junction of diodes 96 and 98. A diode 114 is connected across the solenoid (not shown) of counter 111 to suppress back e.m.f.

The switch units 88 and 104 together with the coil 74 form part of the same relay. Accordingly, the moveable contacts 86 and 106 are ganged together as indicated by the broken line 116 so as to move together. Contacts 86 and 106 are shown in the positions which they assume when coil 74 is energized. Hence when the relay is off, contacts 86 and 106 are closed with respect to contacts 84 and 108 respectively. The counter 111 is of the type that provides a half count in response to the commencement of an input signal and a half count in response to the termination of the input signal, so that a full count, i.e., a count representing one workpiece, is produced only when an input pulse has terminated. This type of operation is common to many solenoid-actuated mechanical counters where the least significant digit display member indexes half a digit when the solenoid is energized and half a digit when it is deenergized. By way of example, the counter may be a Sodeco Counter, Type TCe B65 (24 volts DC) made by Landis & Gyr, Inc. of New York, NY.

Operation of the counter control system just described is as follows: When the press is started by turning on motor 10, power is automatically applied across the terminals 64 to energize the stepdown transformers 66 and 68. Assume that motor 10 is off, probe 40 is ungrounded, and switch 54 is open so that switch contact 86 also is ungrounded, and no power is available at terminals 64 so that relay coil 74 is deenergized and contacts 86 and 106 are closed with respect to contact 84 and 108. Now motor 10 is turned on to start the press and simultaneously power is applied acrossterminals 64. Since probe 40 is ungrounded and since contact 82 is open, the triac remains off and coil 74 remains deenergized. However, when the first part is discharged from the press onto chute 30 and passes the probe 40, it momentarily grounds the latter to produce a pulse at the gate electrode G which causes the triac 76 to fire. In essence, grounding of probe 40 causes a drop in the voltage on the gate electrode sufficient to cause the triac to fire. When the triac fires, current flows through it via coil 74, energizing the latter sufficient to cause the contacts 86 and 106 to open with respect to contacts 84 and 108 and close on the contacts 82 and 110 as shown. As soon as contact 106 opens with respect to contact 108, the counter 111 completes a full count, and completion of the count will be indicated by indicator light 60 being turned on. The relay stays on as long as the triac conducts. lf while the triac is on, switch 54 closes as a result of continued operation of the press, a path is provided for current through contacts 82 and 86 and switch 54. This current path causes the triac to be extinguished since both of its terminals T and T will be at the same, i.e. ground, potential. Furthermore, so long as this current path exists, the relay coil will remain energized and further firing of the triac will have no effect on the counter 111. When switch 54 is reopened, the holding circuit for coil 74 will be opened and the relay contacts 86 and 106 will reclose on contacts 84 and 108 and be operated to provide a half count. The counter will remain in this new half-count position until probe 40 is again grounded, whereupon the triac will fire, the relay coil 74 will again be energized, and the counter will be caused to complete its count. From the standpoint of systems operation, the dropping out of the relay caused by reopening of switch 54 may be viewed as conditioning the circuit so that the counter can respond when the probe 40 is again grounded by a workpiece.

It is to be noted that diode 70 functions as a half-wave rectifier and that diode operates to suppress any back e.m.f. generated in coil 74. The capacitor 78 functions as a filter so that the voltage supplied to triac 76 is substantially DC. Hence, once the triac is turned on by grounding probe 40, it will stay on after the probe 40 is ungrounded until such time as the potential difference between its terminals T and T is reduced to a level insufiicient to sustain conduction.

The circuit described above is designed to prevent an erroneous count in either of the following situations: (a) switch 54 is held closed and probe 40 is repeatedly grounded for short intervals, and (b) probe 40 is continuously grounded and switch 54 is repeatedly closed and opened.

With respect to situation (a), it is to be noted that if the triac is off with probe 40 ungrounded and switch 54 open, the relay coil will be deenergized. If now switch 54 is closed and held closed and probe 40 is grounded momentarily, the triac will fire and thereby energize coil 74. Energization of coil-74 will cause the relay contacts 86 and 106 to shift to the position shown in FIG. 3, with the result that the counter (already in a half-count state) will complete a full count and a holding circuit for coil 74 is established through switch 54 and contacts 82 and 86. As soon as this holding circuit is established-the triac will stop conducting since now its terminal T, will be at the same potential as its terminal T (it is to be noted that triac 76 will stop conducting even if probe 40 continues to be grounded). So long as switch 54 is closed and coil 74 is energized, repeated grounding of probe 40 will have no effect on the triac and the counter will remain in its full-count state.

However, if subsequently switch 54 is reopened, the relay will drop out and the counter will shift by another half count.

With respect to situation (b), if probe 40 is grounded while switch 54 is open, the triac will fire, coil 74 will be energized, contacts 86 and 106 will open with respect to contacts 84 and 108 and close on contacts 82 and 110, and the counter (already in a half-count state) will complete a full count. It then the probe is ungrounded while switch 54 is still open, the triac will stay on, coil 74 will remain energized, and the state of the counter will remain unchanged. However, if the probe is left grounded and now switch 54 is closed, the triac will shut down while coil 74 will remain energized. Hence the counter will remain unchanged. If thereafter, switch 54 is opened while the probe is still grounded, the triac will go on again before the relay can drop out and the counter will remain unchanged. If subsequently, the probe is ungrounded while switch 54 is open, the triac will remain on until such time as the switch 54 is again closed and reopened. Hence no erroneous count.

It is to be appreciated that the triac can fire faster than the relay can drop out or pull in; hence, if probe 40 is permanently grounded and switch 54 is repeatedly opened and closed the triac will extinguish each time switch 54 closes but the relay will not drop out.

It is to be noted also that the resistor 92 is employed to provide bias voltage to terminal T and gate G of the triac when the probe is grounded to assure that the triac will fire when switch 54 contacts 82 and 86 are open. The contact resistence of the probe also must be held within appropriate limits in order to assure firing of the triac. Similarly, the resistance of the path comprising contacts 82 and 86 and switch 54 must be held low in order to assure that the relay will drop out. Without resistor 90, the gate of triac 76 will be floating. With resistor the gate is tied to triac terminal T so as to prevent triac firing by stray potential.

It is to be noted that counter 111 need not be a solenoid-actuated mechanical counter but instead may be a standard pulse-operated electronic digital counter of the type adapted to record or count in response to the trailing edge of each positive pulse. in this connection, it is to be noted that the output of full-wave rectifier 94 is applied to counter 111 as pulses because of the opening and closing of contacts 106 and 108. The pulses applied to counter 111 commence when contacts 106 and 108 are closed and terminate when contacts 106 and 108 are reopened. Although the solenoid-operated counter registers a half count when the input pulse begins, it does not complete a full count until the input pulse terminates. Hence it may be said that counter 111 registers a full count only in response to the trailing edge of the input pulse, much in the same way as an electronic counter. The polarity of the input pulses will, of course, depend upon whether the counter responds to negative or positive signals.

It is also believed to be apparent that since triac 76 is operated unidirectionally, it may be replaced by a silicon-controlled rectifier having its anode connected to coil 74 and its cathode connected to the bottom end of the secondary of transformer 66, while its gate would be connected in the same way as the gate of triac 7 6. However, a triac is preferred since its input voltage firing range is broader than that of an SCR. It is further contemplatedthat the relay shown in FIG. 3 may be replaced by a solid state relay.

Of course the circuit of FIG. 3 may beused with apparatus other than a cyclically operating press as herein described or illustrated and that other sensor means other than probe 40 and switch 54 may be used to generate control signals. For example, such other sensor means may be optically responsive devices such as photodiodes, or transducers responsive to impact or sharp changes in heat, gas pressure, weight, etc.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended thatall matter contained in the above description or shown in the accompanying drawing shall be interpreted in an illustrative and not in a limiting sense.

lclaim: I i

11 A counter control circuit for a counter adapted to record a count in response to a count pulse, said circuit comprising means including a first bistable switch for initiating a count pulse when said first switch assumes a first state and terminating said count pulse when said first switch assumes a second state, a second bistable switch adapted to assume first and second states, each of said switches normally being. in one of its two states, energizeable means operable when energized to switch said switches to the other of itstwo states, a thyristor, means connecting said thyristor to said energizeable means so as to provide a first energizing circuit for said energizeable means when said thyristor is rendered conductive, a first sensor, means connecting said first sensor to said thyristor for triggering said thyristor to conduction when a first predetermined condition is sensed by said first sensor, a second sensor, means connecting said second sensor and said second bistable switch to said energizeable means so as to provide a second energizing circuit for said energizeable means when said second bistable switch is in its said other state and a second predetermined condition is sensed by said second sensor, said second bistable switch being connected to said energizeable means in shunt with said thyristor so that said thyristor is rendered nonconductive by said second energizing circuit, whereby count pulses are generated according to the conditions sensed by said sensors.

2. A counter control circuit according to claim 1 further including indicator means for providing a visual indication each time a count pulse is generated, said indicator means being connected to and controlled by said first bistable switch.

3. A counter control circuit according to claim 1 further including a counter and means for applying said count pulse to said counter.

4. A counter control circuit according to claim 1 wherein said energizeable means is a solenoid, and said solenoid and said bistable switches together comprise a relay.

5. A counter control circuit according to claim 1 wherein said thyristor is a triac having its T and T electrode terminals connected in series with said energizeable means and its gate electrode terminal connected to said first sensor.

6. Acounter control circuit according to claim 5 wherein said energizeable means is a solenoid. I

7. A counter control circuit according to claim 5 wherein said sensor is a conductive probe, and further wherein said first condition is grounding of said probe.

8. A counter control circuit according to claim 5 wherein said means for initiating and terminating said count pulse includes a source of DC potential coupled to said first bistable switch.

9. A circuit for operating a counter of the type adapted to record successive input pulses, said circuit comprising a relay having first and second switches two capable of switching from one to the other of t switching states and a solenoid adapted when energized to cause said switches to shift to selected switching states, a first source of' DC potential, a pair of output terminals, means connecting said first switch to said source of DC potential and said output terminals so that the potential across said terminals is at one level when said first switch is in one of its states and at a second level when said first switch is in the other of its states, whereby with said terminals connected to a counter switching of said first switch from said first state to said second state and back to said first state produces a pulse input to said counter, a second source of potential, means including a thyristor and said second source connected in series with said solenoid for providing a first energizing circuit for said solenoid when said thyristor is rendered conductive, said thyristor having a gate electrode, means including a first sensor connected to said gate electrode for biasing said thyristor to conduction when said sensor is in the first of two states, and meansinclucling a second sensor and said second switch connected in series with said solenoid and said second source for providinga second energizing circuit for'said solenoid when said second switch is in one of its states and said sensor is in the first of two given states, said second energizing circuit when established serving to prevent conduction of said thyristor, whereby said thyristor is rendered conductive only when said first sensor is in its first state while said second sensor is not in its first state and an input pulse to said counter is produced only if said thyristor is rendered nonconductive by said second switch being in said first state and said solenoid is deenergized'by said second switch switching out of its saidfirst state.

10. -A counter control circuit for a counter adapted to record a count in response to the trailing edge of a pulse, said circuit comprising a first bistable means for initiating a counter input pulse when in the first of its two states and for terminating said input pulse when in the second of its two states, second bistable switching means capable of assuming 'either of first and second states, energizeable control means for both said bistable means, means connecting said energizeable control means and both said bistable means so that both said bistable means switch to said second states when said control means is energized and switch to said first states when said control means is deenergized, a first normally open energizing circuit for said energizeable control means including a thyristor, said first energizing circuit adapted to be completed when said thyristor is rendered conductive, first sensing means adapted to trigger said thyristor to conduction when a first given condition is sensed, a second energizing circuit for said energizeable control means, said second energizing circuit including said second bistable means and a second sensor, said second energizing circuit being completed when said second bistable means is in its second state and a second given condition is sensed by said second sensor, said second energizing circuit being in parallel with said first energizing circuit so as to shunt said thyristor when said second energizing circuit is completed, whereby completion of said second energizing circuit renders said thyristor nonconductive so long as said second condition subsists.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION March 9 1971 Patent No. 3,559 I680 Dated Inventor-(s) Clement F. Cox

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, Line 16, "two" is changed to "each".

Column 6, Line 17 "t" is changed to "two".

Signed and sealed this 22nd day of June 1971.

(SEAL) Attest:

EDWARD M.FLETCHx3R,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents 

1. A counter control circuit for a counter adapted to record a count in response to a count pulse, said circuit comprising means including a first bistable switch for initiating a count pulse when said first switch assumes a first state and terminating said count pulse when said first switch assumes a second state, a second bistable switch adapted to assume first and second states, each of said switches normally being in one of its two states, energizeable means operable when energized to switch said switches to the other of its two states, a thyristor, means connecting said thyristor to said energizeable means so as to provide a first energizing circuit for said energizeable means when said thyristor is rendered conductive, a first sensor, means connecting said first sensor to said thyristor for triggering said thyristor to conduction when a first predetermined condition is sensed by said first sensor, a second sensor, means connecting said second sensor and said second bistable switch to said energizeable means so as to provide a second energizing circuit for said energizeable means when said second bistable switch is in its said other state and a second predetermined condition is sensed by said second sensor, said second bistable switch being connected to said energizeable means in shunt with said thyristor so that said thyristor is rendered nonconductive by said second energizing circuit, whereby count pulses are generated according to the conditions sensed by said sensors.
 2. A counter control circuit according to claim 1 further including indicator means for providing a visual indication each time a count pulse is generated, said indicator means being connected to and controlled by said first bistable switch.
 3. A counter control circuit according to claim 1 further including a counter and means for applying said count pulse to said counter.
 4. A counter control circuit according to claim 1 wherein said energizeable means is a solenoid, and said solenoid and said bistable switches together comprise a relay.
 5. A counter control circuit according to claim 1 wherein said thyristor is a triac having its T1 and T2 electrode terminals connected in series with said energizeable means and its gate electrode terminal connected to said first sensor.
 6. A counter control circuit according to claim 5 wherein said energizeable means is a solenoid.
 7. A counter control circuit according to claim 5 wherein said sensor is a conductive probe, and further wherein said first condition is grounding of said probe.
 8. A counter control circuit according to claim 5 wherein said means for initiating and terminating said count pulse includes a source of DC potential coupled to said first bistable switch.
 9. A circuit for operating a counter of the type adapted to record successive input pulses, said circuit comprising a relay having first and second switches two capable of switching from one to the other of t switching states and a solenoid adapted when energized to cause said switches to shift to selected switching states, a first source of DC potential, a pair of output terminals, means connecting said first switch to said source of DC potential and said output terminals so that the potential across said terminals is at one level when said first switch is in one of its states and at a second level when said first switch is in the other of its states, whereby with said terminals connected to a counter switching of said first switch from said first state to said second state and back to said first state produces a pulse input to said counter, a second source of potential, means including a thyristor and said second source connected in series with said solenoid for providing a first energizing circuit for said solenoid when said thyristor is rendered conductive, said thyristor having a gate electrode, means including a first sensor connected to said gate elEctrode for biasing said thyristor to conduction when said sensor is in the first of two states, and means including a second sensor and said second switch connected in series with said solenoid and said second source for providing a second energizing circuit for said solenoid when said second switch is in one of its states and said sensor is in the first of two given states, said second energizing circuit when established serving to prevent conduction of said thyristor, whereby said thyristor is rendered conductive only when said first sensor is in its first state while said second sensor is not in its first state and an input pulse to said counter is produced only if said thyristor is rendered nonconductive by said second switch being in said first state and said solenoid is deenergized by said second switch switching out of its said first state.
 10. A counter control circuit for a counter adapted to record a count in response to the trailing edge of a pulse, said circuit comprising a first bistable means for initiating a counter input pulse when in the first of its two states and for terminating said input pulse when in the second of its two states, second bistable switching means capable of assuming either of first and second states, energizeable control means for both said bistable means, means connecting said energizeable control means and both said bistable means so that both said bistable means switch to said second states when said control means is energized and switch to said first states when said control means is deenergized, a first normally open energizing circuit for said energizeable control means including a thyristor, said first energizing circuit adapted to be completed when said thyristor is rendered conductive, first sensing means adapted to trigger said thyristor to conduction when a first given condition is sensed, a second energizing circuit for said energizeable control means, said second energizing circuit including said second bistable means and a second sensor, said second energizing circuit being completed when said second bistable means is in its second state and a second given condition is sensed by said second sensor, said second energizing circuit being in parallel with said first energizing circuit so as to shunt said thyristor when said second energizing circuit is completed, whereby completion of said second energizing circuit renders said thyristor nonconductive so long as said second condition subsists. 